Interaction of Nitroxide Spin Labels with Chloroplasts

Chloroplasts isolated from oats eliminated the electron spin resonance (ESR) signals from spin labels in white light and partially restored them in far-red light. Only the white light-mediated reaction was blocked by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). In contrast, oat (Avena sativa L. cv. Garry and Park) leaf mesophyll protoplasts oxidized the spin labels in both white and far-red light, with and without DCMU. Light had no obvious effect on spin label motion within chloroplast membranes. The results suggest that, in isolated chloroplasts, nitroxide spin labels may be reduced by photosystem I within the thylakoid bilayer resulting in loss of the ESR signals. The reduced forms may be reoxidized by an element of the photosynthetic electron transport chain which operates between the DCMU block and the photosystem I reaction center. In addition, a light-mediated destruction of the spin labels occurs in both chloroplasts and protoplasts. The reduced form of the nitroxide (i.e. the hydroxylamine) may be resistant to this destruction.